1. Field of the Invention
The present invention relates to a cleaning device and an image forming apparatus.
2. Description of the Related Art
As a cleaning device employed in an image forming apparatus such as a copying machine, a facsimile, and a printer, known is a blade cleaning method for removing a toner on an image carrier by pressing a cleaning blade made of an elastic member against the periphery of the image carrier that is a body to be cleaned and by scraping the toner off. The blade cleaning method is widely used because of its simple implementation and stable performance.
In recent years, there has been a growing demand for the improvement of image quality, and in response to the demand, the formation of toners having a smaller particle size and being more spherical proceeds. Higher precision, higher resolution, and higher definition images can be obtained by reducing the particle size of a toner, and the developing performance and transfer performance can be improved by making the toner more spherical.
However, when a toner that has a smaller particle size and is made more spherical is used, it becomes difficult to perform favorable cleaning by a typical cleaning blade method. The reason of this is described below. That is, a cleaning blade removes a toner while rubbing the surface of an image carrier, which causes a so-called stick slip caused by the deformation of the edge of the cleaning blade due to frictional resistance against the image carrier. Therefore, a minute space is formed between the image carrier and the cleaning blade. A toner having a smaller particle size is prone to enter this space, and the entering toner being more spherical in shape is prone to roll in the space because angular moment is generated in the toner. As a result, the toner that has a smaller particle size and is made more spherical is prone to push the cleaning blade up and to get into a space between the cleaning blade and the image carrier.
The use of the toner that has a smaller particle size and is made more spherical is considered to increase the pressing force (linear pressure) of the cleaning blade against the image carrier to inhibit the toner from getting inside. However, when the pressing force is increased to impose high loads, the abrasion of the image carrier and the cleaning blade proceeds to severely shorten their service life. Devices are required to have long service life in recent years, and therefore, such disadvantage associated with durability needs to be avoided.
When an electrostatic cleaning method is employed as with cleaning devices disclosed in Japanese Patent Application Laid-open No. 2002-202702 and Japanese Patent Application Laid-open No. 2007-25173, a toner produced by a polymerization method can also be favorably removed. Even when the charged polarity of the toner to be removed is both positive and negative, the toner can be favorably removed. The cleaning device disclosed in Japanese Patent Application Laid-open No. 2002-202702 includes a conductive blade that comes in contact with an image carrier and is applied with a voltage having a reversed polarity relative to the polarity of a cleaning brush, as a polarity control unit that aligns the charged polarity of the toner. The conductive blade is provided upstream of the cleaning brush as a cleaning member. In the cleaning device disclosed in Japanese Patent Application Laid-open No. 2002-202702, while a toner left untransferred passes through a position (blade contact position) where the conductive blade makes contact with the image carrier, charges are injected to the toner from the conductive blade. In such a manner, for example, the charged polarity of the toner is aligned with the same polarity (typically, the normally charged polarity of the toner) as that of the conductive blade. Accordingly, the charged polarity of the toner that passes through the blade contact position and that reaches a position (roller contact position) where the cleaning brush comes in contact with the image carrier is one of the polarities (the same polarity as that of the conductive blade). Therefore, even when a toner having a positive polarity and a toner having a negative polarity are mixed before cleaning, the cleaning brush can electrostatically recover the toners.
The cleaning device disclosed in Japanese Patent Application Laid-open No. 2007-25173 includes a first cleaning brush applied with a voltage having a reversed polarity (positive polarity) relative to a normally charged polarity of a toner, and a second cleaning brush applied with a voltage having the same polarity as the normally charged polarity of the toner at the downstream of the first cleaning brush. The toner having a normally charged polarity (negative polarity) on an image carrier is electrostatically adsorbed to the first cleaning brush serving as a normally charged toner cleaning member and is removed from the image carrier. The toner having a reversed polarity (positive polarity) relative to the normally charged polarity on the image carrier is electrostatically adsorbed to the second cleaning brush serving as a reversely charged toner cleaning member and is removed from the image carrier. Accordingly, both the toner having a positive polarity and the toner having a negative polarity can be removed from the image carrier.
However, when an untransferred toner image, such as a toner pattern, in which a large amount of toner adheres to the image carrier enters a cleaning device having such structure disclosed in Japanese Patent Application Laid-open No. 2002-202702 and Japanese Patent Application Laid-open No. 2007-25173, the toner cannot be favorably removed from the image carrier. This may cause cleaning failure.
For this reason, the applicant of the present invention has developed the following cleaning device described in Japanese Patent Application No. 2009-293120. Specifically, the cleaning device includes a pre-cleaning brush that roughly removes a toner having a normally charged polarity. The pre-cleaning brush is provided at a position upstream of the polarity control unit in the movement direction of the surface of the image carrier in the case for the cleaning device disclosed in Japanese Patent Application Laid-open No. 2002-202702. The pre-cleaning brush is provided at a position upstream of the first cleaning brush in the movement direction of the surface of the image carrier in the case for the cleaning device disclosed in Japanese Patent Application Laid-open No. 2007-25173. The pre-cleaning brush is provided in such a manner. Thus, when an untransferred toner image enters the cleaning device, the pre-cleaning brush roughly removes the toner having a normally charged polarity that is dominant in the toner constituting the untransferred toner image. This reduces the toner amount entering a polarity unit and a cleaning brush that are provided downstream of the pre-cleaning brush. Therefore, the structure downstream of the pre-cleaning brush in the movement direction of the image carrier can favorably remove the remaining toner that remains unremoved by the pre-cleaning brush incorporated in the structures in the cleaning devices disclosed in Japanese Patent Application Laid-open No. 2002-202702 and Japanese Patent Application Laid-open No. 2007-25173.
The cleaning device developed by the applicant of the present invention includes a pre-cleaning unit including a pre-cleaning brush and a cleaning unit including a cleaning brush arranged downstream of the pre-cleaning unit in the movement direction of the image carrier. The pre-cleaning unit includes, besides the pre-cleaning brush, a pre-recovery roller as a pre-recovery member that recovers a toner adhering to the pre-cleaning brush, and a pre-scraping blade as a pre-scraping member that makes contact with the surface of the pre-recovery roller and scrapes off the toner remaining on the pre-recovery roller from the pre-recovery roller. The cleaning unit at the downstream also includes, besides the cleaning brush, a pre-recovery roller as a recovery member that recovers the toner adhering to the cleaning brush, and a scraping blade as a scraping member that makes contact with the surface of the recovery roller and scrapes off the toner remaining on the recovery roller from the recovery roller.
In the cleaning device developed by the applicant of the present invention, the toner is adversely fixed to the scraping blade of the cleaning unit at the downstream. When the toner is thus fixed to the scraping blade of the cleaning unit at the downstream, the scraping ability decreases to make the toner remain on the recovery roller. When the toner remains on the recovery roller, the toner on the cleaning brush is unlikely to adhere to the recovery roller, which reduces the toner recovery ability of the recovery roller. When the toner recovery ability of the recovery roller decreases, the toner remaining on the cleaning brush increases to reduce the toner amount that newly adheres to the cleaning brush. As a result, the cleaning ability of the cleaning unit at the downstream decreases, which may cause cleaning failure.
As a result of assiduous research intended to overcome the toner being fixed on the scraping blade of the cleaning unit at the downstream, the inventors of the present invention have found out the followings. That is, a certain amount of a toner enters a contact portion between the scraping blade and the recovery roller, and the entering toner serves as a lubricant to suppress the friction between the scraping blade and the recovery roller. However, in the cleaning device as mentioned above, the pre-cleaning unit removes a large amount of the toner on the image carrier, and therefore, the amount of the toner to be removed by the cleaning unit provided downstream of the pre-cleaning unit is small. The toner amount entering the contact portion between the scraping blade and the recovery roller of the cleaning unit at the downstream is small, and therefore, the toner cannot sufficiently function as a lubricant. Thus, the contact portion of the scraping member generates heat due to the friction between the scraping blade and the recovery roller. As a result, the toner entering the contact portion between the scraping blade and the recovery roller is melted by the heat of the scraping blade caused by the heat generation of the scraping blade and is fixed to the scraping blade.